1. Field of the Invention
This invention relates to gas turbine engines, and more particularly to means for confining cooling air to a flowpath extending about the interior of the outer case of such an engine.
2. Description of the Prior Art
A gas turbine engine has a compression section, a combustion section and a turbine section. The turbine section has a rotor assembly and a stator assembly. An annular flowpath for working medium gases extends axially through the engine. The annular flowpath passes in alternating succession between components of the stator assembly and components of the rotor assembly. The rotor assembly includes a plurality of outwardly extending rotor blades. The rotor blades extend into the working medium flowpath and into proximity with components of the stator assembly. To confine the working medium gases to the working medium flowpath a plurality of outer air seal segments radially oppose the tips of the rotor blade. The outer air seals are part of the stator assembly. An outer case and support structure extending inwardly from the outer case support and position the outer air seals about the tips of the rotor blades.
Because the outer air seals, the outer case, and the rotor blades expand and contract at different rates in response to changes in temperatures of the hot working medium gases, the clearance between the tips of the rotor blades and the outer air seal varies. To minimize the clearance during steady-state conditions such as at cruise, cooling air is discharged against the outer case from cooling tubes circumscribing the case to cause the case to contract. The contracting case displaces the outer air seals inwardly to a smaller diameter. The inward movement of the outer air seals decreases the clearance between the rotor tips and the outer air seals with a concomitant beneficial effect on engine efficiency. One such construction directed to such a structure is shown in U.S. Pat. No. 4,069,320 to Redinger et al. entitled, "Clearance Control For Gas Turbine Engine".
In modern engines, cooling air is also flowed through passages on the interior of the case. The cooling air removes heat from the case and from the outer air seals which are in intimate contact with the hot working medium gases to increase the service life of such components. Along the cooling air flowpath, the cooling air is at a higher pressure than the surrounding gases. The case forms the outer boundary of the cooling air flowpath and seal means extend between the cooling air flowpath and the hot gases to form an inner boundary of the flowpath. Holes through the seal means face a corresponding cavity in the outer air seal and precisely meter the flow of cooling air into the cavity. Cooling air leakage around the edges of such seal means degrades engine performance. One example of a design directed to a construction which meters cooling air to outer air seal cavities and which blocks the leakage of cooling air around the ends of the seal means is shown in U.S. Pat. No. 3,583,824 to Smuland entitled, "Temperature Controlled Shroud and Shroud Support". The seal means of Smuland is welded or brazed to the outer air seal. U.S. Pat. No. 3,836,279 to Lee entitled, "Seal Means for Blade and Shroud" discloses a circumferentially extending sheet metal shroud seal. The seal has a plurality of openings each of which faces a corresponding cavity in an outer air seal. A raised portion extends around each opening and is resiliently deformed to provide an annular seal around the opening.
Notwithstanding the above art, scientists and engineers are still seeking to increase the sealing effectiveness of a seal means extending about the interior of an engine case between an outer case and the hot working medium gases and, in particular, between the outer case and an array of outer air seals.